tap-dance: Support for holding keys

tap-dance: Support for holding keys

With this change, tap dance will now store the pressed state of the
tap-dance key, and allow one to make an action sooner, while the key is
still held, and only unregister when the key is released.

The registration must happen in the `on_dance_finished` callback, while
unregistering goes to `on_reset`. The surrounding code makes sure not to
call either multiple times.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>